This invention relates to a particle size analyzer and more particularly to an apparatus and method for determining the particle size distribution in dry powders by charging the particles and measuring the individual charge accumulated on each particle in a sample.
Characterizing particles that make up bulk materials has become increasingly important as the sophistication and control requirements of industrial processes increase. In the past the usual method was to use a microscope and measure the size distribution of an exemplary sample of the material to obtain an indication of the mean particle size and size distribution.
More recently a number of instruments have been developed to avoid the tedious and labor intensive process of measuring and counting particles under the microscope, by apparatus which can provide a particle size measurement and particle size distribution analysis automatically.
The range of particle size measured, particularly of dry powders, typically has a mean diameter between 1 xcexcm and 10 xcexcm. In most instances the instruments used employ indirect measurement techniques which include sieve fractionation, laser light scattering, dynamic light scattering, sedimentation/centrifugation, and gas absorption.
More recently, in an article published in the Proceedings of the 7th International Metrology Congress, Oct. 17-19, 1995, Niemes, France, pp. 215-217, entitled xe2x80x9cAn Electrodynamic Method Of Obtaining Calibration Materials For Granulometryxe2x80x9d, authored by V. J. Gerasimov and V. V. Romanenko, and incorporated herein by reference, it was shown that it is possible to obtain narrow-disperse calibration powders by using electrodynamic sedimentation of powders. In this article it is proposed that dry powder may be divided by particle size by subjecting the powder to a charging DC voltage applied between two inclined angled electrodes and providing a number of openings along a bottom electrode and an ancillary air stream of controlled velocity. By controlling the inclination of the bottom electrode, particles are separated along the electrode by mass and can be collected at specific points to obtain particles of selected similar mean diameter.
Most of the above systems, except for systems relying on the size measurement of the actual particle under a microscope, typically segregate a number of particles and calculate an average diameter for the segregated particles. Others use a means of fitting a presumed particle size distribution to a size sensitive response such as in the laser light scattering methods.
The present invention provides an instrument for automatically charging and obtaining a direct measurement of a dry powder""s particle charge distribution in a sample, which uses electrodynamic particle separation in a new way in combination with an electrometer to obtain a measurement of the individual, particle-by-particle, charge on a plurality of charged particles, and to present an analysis of the charge distribution. The charge on each particle is related to the particle surface area and thus is also an indirect indication of the particle equivalent sphere or cube diameter. This process has the advantage that individual particles are measured and the individual particle distribution is obtained, rather than providing an average particle area or volume measure or a fit to a response function indirectly reflecting the particle size distribution.
In accordance with the present invention, there is provided an apparatus for determining the particle size distribution of a plurality of particles in a sample, the apparatus comprising a particle charging chamber adapted to charge the particles to a saturation level proportional to a dimension of the particle and to release particles from the charging chamber primarily one-at-a-time, a collector electrode located outside the charging chamber, and a charge-measuring device connected to the collector electrode for producing an output with a value representing the measured charge. The particle charging chamber may also be a pseudo-fluidization chamber such as, for example, an electrodynamic pseudo-fluidization chamber. Such a particle charging chamber comprises a top and a bottom electrode therein, a DC voltage source connected to the top and bottom electrodes, and a conversion device for receiving the output from the charge-measuring device and for converting measured individual particle charge data to a particle surface area distribution. The bottom electrode inside the particle charging chamber is spaced from the top electrode and has an exit orifice, an inner surface facing the top electrode, and an outer surface opposite the inner surface. The particle charging chamber further comprises a side wall extending between the top and bottom electrodes and a means for inserting the plurality of particles in the chamber. The collector electrode is aligned with the bottom electrode exit orifice and is spaced therefrom. The DC voltage source applies a voltage to the top and bottom electrodes of a magnitude insufficient to cause arcing between the electrodes.
The apparatus may further include an enclosure defining a space enclosing the particle charging chamber and collector electrode. This space may contain a gas, in particular an inert gas such as nitrogen, at a pressure above atmospheric pressure.
The chamber may be subdivided into subchambers by a plurality of middle electrodes located between the top and bottom electrodes and spaced therefrom, for example a single middle electrode may divide the chamber into an upper section and a lower section.
Such a middle electrode may have a convex surface with an apex facing the top electrode. The orifice in the middle electrode is placed away from the apex of the convex surface. The DC voltage source may apply a first DC voltage between the middle electrode and the bottom electrode and a second DC voltage between the top and middle electrodes, the DC voltages insufficient to produce arcing between the electrodes.
The apparatus may further include one or more fan-shaped light beams, such as two beams in parallel spaced planes between the exit orifice and the collector electrode, and a means for detecting any beam disruption by the passage of a charged particle and for generating an electrical signal indicating the passage and time of passage of any such particle through each of the beams of light.
In accordance with the apparatus of the present invention, there is also provided a method for determining a particle size distribution of a plurality of particles having a plurality of sizes by charging the plurality of particles and measuring a charge magnitude for each particle. The method comprises the steps of charging each of the particles in a particle charging chamber to a saturation level proportional to a dimension of the particle and releasing the particles from the chamber primarily one at a time. For example, the particles may be introduced in a space between a pair of electrodes in the presence of a static electric field wherein the method further includes providing an orifice in one of the pair of electrodes. The method also comprises collecting any charged particles exiting the particle charging chamber, such as through the orifice, and generating an electrical signal having a magnitude indicative of the charge of any such escaping charged particle; and using the distribution of the electrical signal magnitudes of a plurality of charged particles as an indicator of the particle surface area distribution.